An Anatomically Accurate Analogy
by TheBookshelfDweller
Summary: "The analogy I hear most when people talk about us is that we are like the brain and the heart. However, the reason why I think it is an apt simile is not the same reason most people have in mind." By now, John Watson and Sherlock Holmes have virtually become synonymous with the two organs so indispensible for living. This is a different take on the reason why this is so.


**Author's note: I am very grateful to everyone who left reviews or favourited my other stories - you are my inspiration and you make my day :) Special thanks, again, to Isayan who's continuing support is overwhelming.**

**This story was my attempt at putting two semesters of Basics of Neurology and Anatomy&Physiology to some good use - and what better use is there than fanfiction? Any and all mistakes are mine, and if there are any I just hope my professors don't spend their free time reading fan fiction, or they might revoke my grade :P**

**It's time for another disclaimer - guess who is neither Steven Moffat, Mark Gatiss nor Sir Arthur Connan Doyle, and as such is not entitled to claim anything Sherlock-related as property? That's right, I'm not!**

**Enjoy your reading! :)**

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The analogy I hear most when people talk about us is that we are like the brain and the heart, you being the former and I the latter. It is definitely overused, but I guess that's because it is very appropriate. However, the reason why I think it is an apt simile is not the same reason most people have in mind. When they call us the brain and the heart they usually mean to say that while you are the one in charge of the mental work, the thinking, I am the one responsible for sentiment. You are cognition, while I am emotion. I'm not saying that is wrong, but it is not completely true, either. Yes, you do tend to outshine me in the thinking department with your deductions and speed-of-light connections, while I most certainly have a closer relationship with my feelings than you do, and much less problem expressing some of them. Well, I do say _some_.

However, it is true that I, myself, can think, too. In fact, I think a lot. I'm not an idiot (you would disagree, but I mean idiot of the Andersonian variety, which I am definitely _not_), I do, in fact, have a medical degree, which isn't all that easy to earn. Medical school involves quite a lot of thinking, you know.  
It is also true that despite your earnest efforts, you are not completely immune to emotions. In fact, I think you would enjoy explaining the fault in their logic to everyone who romanticises the heart into being the organ in charge of feeling, seeing as feelings are products of chemical reactions that occur within the brain. You would enjoy proving them wrong, that is until your own logic caught up with you and you realised that meant that, if you were the brain in the analogy, you were – by your own facts, no less – _very_ capable of emotion. Oh, I would pay to see your face if that ever happened.

But to return to my original thread of thought; the reason why I think the brain-heart comparison is a befitting one is because these two organs are in a symbiosis much alike the one enjoyed by the two of us. It has nothing to do with emotion or cognition. We are both (almost) equally capable of both. It has to do with provision of elements indispensible for the proper functioning of both organs. Food, oxygen, disposal of toxic compounds – the heart; stimulus, change of rhythm, variation – the brain.

The heart pumps nutrient-carrying blood to the brain - I feed you, quite literally. Christ, sometimes I practically _force_ feed you, just to get you to ingest anything. There are multiple blood vessels netting around the brain, encasing it in a web of tissue. Arteries that run up from your chest, via the neck and through the hollows in your skeleton, only to weave each other into the Willis' circle at the base of you skull, bring fresh blood, filled with sustenance. I am blood, coursing ceaselessly, reminding you to _eat_, to _sleep_, and spilling red when you fail to do so. I am your constant reminder of mortality, of the gossamer nature of your transport.  
The arteries also bring the vital molecules of oxygen in order to prevent brain tissue from decaying. I like to think that every time you don't light a cigarette because of my adamant refusal to tell you where I've hidden the emergency pack, I am helping you breath. I provide your brain with the indispensible gas, however indirectly.

Then there are veins, worming through the crevices and expanding to form sinuses, absorbing all the toxic produce of the brain's metabolism and draining it away. They keep the brain from suffocating in the destructive side-effects of its brilliant work. The nicotine, the drugs, your absolute carelessness regarding your welfare – I drain them away when they accumulate to a dangerous level. Blood carries it all back to the heart, pass the lunges and onwards, to be cleansed. I take your vices and your demons, the ones you allow me to, and shove them away. Old blood out, new blood in. Red-stained ablutions.

Blood is the worker, the keeper, the care-taker. It will never have the speed of an impulse or the sophistication of a synapse, of neurotransmission. But blood is indispensible, one can't afford to lose too much of it before it has to be replaced.

Of course, there is the rather obvious and physical aspect of this analogy, the part pertaining to the fact that the heart is a muscle. While I might not be as fit as I was while in active service, I did save your scrawny neck using only physical strength quite a number of times. See? Even that fits the metaphor.

Then there is you. You are electrifying, jumping around like an impulse down an axon of a nerve. Your EEG would probably be your most accurate portrait, and if your brain were to be scanned, it would light up in ways unseen before, only to form an abstract painting of unparalleled beauty. Your thinking is an art, and it would show as such. You give the term _brain activity _a whole new definition.

So, right, yeah – if I had to compare you to an organ, I would compare you to the brain, just like everyone else. The brain is a master at multitasking, it is bound to be if one is to live and function properly. It can run several functions at once and store almost infinite amount of data. There are parts of it that never rest, vital parts that cannot afford a break from the routine – the eternal insomniac. Now, who does that remind me of?

The brain is connected to the heart via _nervus vagus_, the vagus nerve, which controls the heart rate. There are multiple bungles of sympathetic and parasympathetic nerves traversing to and from the heart, regulating its tempo, speeding it up or placating it, according to need. When the brain is in danger, the heart beats faster. When you are in danger, I run faster. It's an integral part of the fight-or-flight response – it's instinctual, a reflex. The brain can rile up the heart, and heaven knows you rile me up in ways I would have previously thought impossible. But if the brain is calm, then so is the heart.

The brain provides the heart with stimulus, one saying "excitement" or "danger", the other relaying "rest", "contentment". You shoot bursts of electricity at me, through me, speeding me up, riling me up, then calming me down. You permeate me with nerve fibres the same way I encase you in blood vessels. Arteries and axons, muscle and myelin.

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Another problem with the usual use to the heart-brain metaphor is that people assume we would simply cease to exist without each other. Well, not that much "cease to exist", but break, fall apart, cease to function properly. As if there is some invisible plug that gets pulled if one of us disappears or dies, causing the other one to shut down. Well, there is not.

In fact, we could both go on without each other. A heart can still beat if the nerves connecting it to the brain are severed, but it beats in monotone. And if the brain is damaged beyond repair, if it is proclaimed dead, the heart can still be kept beating by machines, eventually being implanted into a new host. The brain can live if a heart ceases to function properly, because hearts can be replaced. A transplant, a new heart to feed the brain all that it needs, and the brain goes on living.

Of course, it wouldn't be the perfect symbiosis, because no organ matches the body as well as the original one. It happens sometimes that the body rejects a new organ despite a seemingly apparent compatibility. I wonder if at times the organ can refuse the body. It would manifest the same way, but I feel that somehow the semantics of it makes a difference.

So you see, we could go on living without each other.

If you died I would be kept alive in the husk of the life that used to contain you, by people who care for me, who need me, just long enough so I can be transplanted into a new life, one with another amazing person in it. Many amazing people, actually. But it would never be the same. I would be forced to forever beat in monotone, at a steady, boring pace. I would become a blood-pumping litany.  
I could survive your death again. I already have, once. My new body would need me, the people around me would mourn me if I gave up. I could pump some other blood through some other veins, blood compatible enough not to destroy me. I could.

But I think that, if it came to that, this time I would choose to reject the body.

You could definitely survive without me; you could even continue to thrive. A heart is by no means irreplaceable. If compatible enough, any heart will do a good enough job of feeding you, providing you with the basic substances required for physical survival. However, that doesn't mean it would ever feel completely right. But the brain can ignore that. Delete the fact the heart sits just a bit uncomfortably in the chest, and trade the feeling for the fact that it staves off the collapse which would be imminent in its absence. Still, no matter how efficient the new heart may be, the body still may reject it. It is a stupid move, rejecting a perfectly good heart, and the underlying processes and reasons are still not fully understood. But my guess would be that in your case, the brain would somehow have something to do with it, even if, medically speaking, it makes no sense.

The heart can live without its original brain, and the brain can live without its original heart. Whether they choose to do so, is another matter entirely.

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**If anatomy was taught in terms of Sherlock and John, I would crash the lessons long after I have passed the subject itself.**


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